2.1 daylighting h3

7/24/2019 2.1 Daylighting H3

1/18

Daylight in Interiors 97________________________________________________________________________

________________________________________________________________________

Interior Lighting Design - A Student's Guide KK/KO'C 97

4. DAYLIGHT IN INTERIORS

4.1 Introduction:

No modern analysis of interior lighting would be complete without considering

the penetration of daylight into interiors.

Until the 1950s, buildings were designed to allow natural light reach virtually all

parts of the building interior. With fluorescent lamp development and cheap

energy, taller deeper plan buildings grew in popularity particularly in city centres

where land was extremely expensive. In addition, air and noise pollution made it

necessary to keep windows closed and provide air conditioning.

The energy crisis of the 1970s made designers think again. In the 1990's public

awareness of environmental issues has made engineers even more aware of the

importance of conserving energy.

Sick building syndrome(SBS) was another factor which moved designers away

from wholly artificial indoor environments. Daylight deprivationis a significant

factor in people's dissatisfaction with buildings and hence SBS. Research shows

that people value the variety of daylight, enjoy the presence of sunlight in a

building and want some view of the outside world.

Buildings of the 21st century are likely to rely more on natural daylight, with

supplementary artificial lightingbeing automatically controlled but with local

override facilities provided for staff.

There are 3 reasons to provide daylight:

1. Healthier and more satisfactory indoor environment;

2. Economic advantage in energy savings;

3. Conserves earths resources and improves company's Green

Image.

4.2 Visual Comfort

Daylight is the natural light to which the human eye has become adapted over

millions of years. In particular colour rendering by daylight is the subjective

standardby which we normally measure the colour performance of an electric

lamp.


2/18

Daylight in a building, even though it is constantly changing in intensity and

colour, will provide very good colour rendering. Where exact colour rendering

of a colour critical task is necessary however, the variation of daylight may cause

difficulties. For this reason, colour matching lamps are used in this case and

daylight is excluded. The colour critical task is normally carried out in a test

room.

The natural variation of daylight in intensity and colour over time provides

variety and interest in interiors. When side windows are used, good modelling is

provided by the cross vector of daylight. An outside view provides information

on the external climate, this provides psychological benefit and allows our

metabolic rhythms to synchronise with the time of day.

4.3 Daylight Quantified

Current work on a European daylighting design guide suggests that there will bea necessity for about 30 daylighting design zones for Europe.

Daylight varies with latitude, season, coastal or inland location, climate and air

quality. The Building Research Establishment in Britain (BRE) provide

information on illuminance due to daylight in London. This shows that mean

horizontal diffuse illuminancevaries from 35 kilolux at midday in summer to

about 10 kilolux at midday in winter - assuming an unobstructed sky (buildings,

trees etc.,).

It is common practice in the UK and Ireland for lighting designers to use a figureof 10 kilolux for external illuminance when calculating the level of illuminance

due to daylight. A minimum of 10 kilolux will be provided for about 70% of

the working day at a latitude of 52% N, which is approximately the latitude

of the British Isles

The amount of daylight received within a building depends on its orientation, the

presence of obstructions and the reflectance of adjacent structures. The area in a

room to which daylight will be considered to contribute significantly to task

illuminance extends to about twice the window height - provided glass is clear,

there are no obstructions (inside or outside) and the window sill is notsignificantly higher than the working plane. (Clear glass in high windows

however, is likely to cause problems of glare.)

It is important to remember that "daylight" is considered to be diffuse light

provided by the sky as a whole and not direct sunlight. The term "skylight" is

often used instead of "daylight".


3/18


________________________________________________________________________


4.4 Energy Cost.

The cost of energy for artificial lighting is a substantial part of the total energy

for most buildings. Typically, light energy would be between 20% and 50% of

the total energy bill.

In HI-TECH air conditioned offices with widespread use of computers, the airconditioning load is nearly always a cooling load. Heat generated by artificial

lighting contributes to this cooling load.

When lighting levels are reduced, there is a reduction in the mechanical cooling

load for the building as well as the electrical load. In some cases, automatic

control of artificial lighting combined with good daylight penetration may even

tilt the balance between the need for air conditioning or natural ventilation for

some buildings.

The substitution of daylight for artificial light can produce savings of 30% -70%* if artificial light is well controlled. Care must be taken however, to shield

occupants from direct effects of sunlight by providing blinds or other shading

devices which can be controlled by occupants.

*Source: Daylighting in Buildings issued by the European Commission

Directorate-General for energy, as part of The Thermie Action Programme.

4.5 Daylight Factor.

Due to the constant variation of daylight, calculations are normally based on apercentage daylight factor. Daylight factor is the amount of daylight reaching an

interior as a percentage of the external illuminance.

Example 4.1

The daylight factor at 3 points A, B and C

in an office are 5%, 3% and 2%

respectively. Calculate the illuminance at

each of these points assuming an external

horizontal illuminance of 10,000 lux.

5% of 10,000 lux = 500 lux

3% of 10,000 lux = 300 lux

2% of 10,000 lux = 200 lux

A B C5% 3% 2%

500 300 200

lux lux lux

Fig. 4.1


4/18

Studies have shown that staff react negatively to sudden interruption of artificial

lighting. If artificial lighting is switched off when the contribution from daylight

is less than twice the illuminance provided by artificial lighting, there is likely to

be complaints from staff. Shadows in the interior and high contrast areas around

windows are created which may lead to irritation of staff in that area. As people

become more accustomed to increased daylight in buildings however, their

tolerance for illuminance and luminance variation is likely to increase.Nonetheless in Northern Europe, daylight will normally be supplemented with

artificial light for most of the day.

4.6 Control of Artificial Lighting

The ideal control system will modulate artificial light levels in each area with the

level of daylight. Dimming should operate very slowly in response to increasing

daylight. In this way transient variations in daylight are ignored and people

working in the area will not notice the artificial light level decrease.

Automatic switching of lights as daylight levels increase is likely to be annoying

to staff; manual switching by staff in the area is acceptable however, because

they feel they have control. If artificial lighting is reduced to 20% output when

daylight levels are high then occupants will have the impression that artificial

lighting is on and they will not feel any sense of deprivation.

Example 4.2

Artificial lighting in an office provides an

average of 500 lux on the working plane.Daylight provides 500 lux at point A, 300

lux at point B and 200 lux at point C. The

lighting over point A is dimmed to 20%

output, the lighting over point B to 50%

output and the lighting over point C to

60% output calculate the total illuminance

at A, B and C.

Illuminance

due to Daylight Artificial Light Total

at A 500 lux 100 lux 600 lux

at B 300 lux 250 lux 550 lux

at C 200 lux 300 lux 500 lux

A B C

20% 50% 60%

500 300 200

Floor

Ceiling

Fig. 4.2


5/18


________________________________________________________________________


Example 4.3

An office area is 1000 m2. It caters for 100 personnel at an average salary of

15,000 per annum. The installed lighting load is 20 Watts per m2and operates

for 3000 hours per annum. Energy costs 10p per kWh.

Calculate:

(a) The annual light energy bill.

(b) The saving if artificial light is reduced to an average of 60% output

throughout the year.

(c) The annual salary bill for the company.

(a) 20 Watts/m2 x 1000 = 20 kW

Energy consumed per annum = 20 kW x 3000 hrs = 60,000 kWh

Cost = 60,000 x 10p = 6,000 p.a.

(b) Average Output = 60%

Average Saving = 40%

Cost benefit = 6,000 x 40% = 2,400 p.a.

(c) Salary Bill = 15,000 x 100 = 1,500,000

The benefit of introducing energy control is significant at 2,400 per annum.The benefit must be kept in perspective however, because the total light energy

bill is 0.4% of the salary bill.

It is clear from this that any annoyance to staff must be eliminated for a control

system to be of benefit to a company. It is equally clear that it is good value to

provide a good quality lighting scheme which staff will appreciate, because a 1%

improvement in productivity will pay for even a top quality lighting scheme in a

short time.

People have become more green in their attitude to the use of energy and ingeneral staff do not like wasteful lighting systems. Usually they want a good

quality scheme and automatic control with override facilities.


6/18

4.7 Daylight Control

Itis necessary to reach a reasonable compromise between ensuring good daylight

penetration and reducing the negative aspects of sunlight. High daylight factors

enable reductions in light energy costs but glare must be controlled and people

must not be subjected to direct sunlight to the extent that their thermal comfort isaffected.

Lightwells, lightshelves, roof lights, roof monitors, lightpipes, lightducts and

transparent insulation are all used to enable daylight to penetrate deeply into

buildings.

Blinds, shades, overhangs, darkened or reflected glass are all useful aids to

control daylight. Special consideration of the selection and positioning of

display screen equipment is necessary in buildings with large arteas of

fenestration (windows) (see chapter 6)

4.7.1 Atria

These are used in modern buildings as inhabited lightwells. One of the aims with

an atrium is to reduce space heating load whilst improving daylight penetration

into the building. How successful it will be will depend on its orientation,

geometry, internal reflectances and the nature of the roof and glazing. An atrium

acts as a thermal buffer and windows facing into it may be larger than they may

otherwise have been. In cities it may be possible to open a window onto an

atrium where it would not be possible to open one onto a street because of noiseand/or air pollution.

4.7.2 Light Shelves

Light shelves are increasing in popularity

in modern buildings.

Lightshelves are placed at the window

above eye level. Incoming daylight is

redirected onto the ceiling improvingdaylight factor at the inner part of the

room. The lightshelf also provides shading

from direct sunlight to people close to the

window. (See Fig. 4.3)

Floor

Ceiling

Fig. 4.3

Light shelf


7/18


________________________________________________________________________


4.7.3 Roof lights

Roof lights are best for daylight

penetrationwhilst minimising heat loss.

Horizontal rooflights admit 3 times

more daylight than vertical windows of

a similar size. In addition, light is castin a more uniform way. The

disadvantage of roof lights is that they

admit more light and heat in summer

than in winter. For this reason, vertical

or near vertical roof lights as well as

roof monitors or clerestorys are often

used instead of horizontal roof lights.

4.7.4 Lightpipes and Lightducts

Sunlight is collected by heliostats(mirrors controlled by tracking devices),

concentrated by means of mirrors or

lenses and then directed to the core of

the building through shafts or acrylic

rods or fibre optic cables. They are cost

effective only in regions where blue

skies are guaranteed for most of the

year. Energy efficient back up lamps

may be substituted for sunlight during

overcast conditions. Recentdevelopment of thermo hydraulic

tracking systems powered by solar cells

should improve the viability of these

devices in the future.

4.7.5 Transparent Insulation

Materials (TIM)

These materials are translucent rather

than transparent and are used mainly as

insulating materials for wall structures.They reduce heat loss from the building

whilst permitting solar radiation to reach

a heat storing inner leaf. Light

transmission varies from 45% to 80%

and it costs about three times the price

of double glazing.

Roof Monitors

Clerestorys

Fig. 4.4

Light ducts

Fig. 4.5

T.I.M.

Fig 4.6


8/18

4.7.6 Daylight Controlling Devices

The type, size and positioning of any shading device will depend on latitude,

building orientation and climate. External shades are the most effective in

reducing heat gains. Interior shades only protect the occupant from direct

sunlight and glare. Internal surfaces absorb the sunlight thus increasing thedemand on the cooling system. Internal shades are cheaper, however, and cost

less to maintain. Adjustable horizontal louvres (venetian blinds) with a specular

finish on the upper surface, can be angled to redirect sunlight in the same manner

as light shelves. this ensures protection of people near windows from direct

sunlight, whilst increasing daylight penetration into the inner parts of the

building. These blinds provide control locally to occupants and they are the most

popular choice in Northern Europe.

4.8 Overall Design

Daylight and artificial light must not be considered in isolation to the other

energy using aspects of a building. A building which allows high daylight

penetration will also have high solar gains. A saving in artificial lighting energy

may be negated by an increase in air conditioning cooling load. The Architect,

Electrical and heating/ventilating engineers and the rest of the design team must

operate as an integrated team producing a comfortable low energy building in

which occupants feel they have adequate control of their environment.

4.9 Depth of a Room.

A well established rule ofthumb for assessing the area

in a room which will have

acceptable daylight is to

determine the "No Skyline

Point". This is the point at

which the skyline is no

longer visible (see Fig. 4.9).

All points further from the

window are not considered

to have acceptable daylight

Desks A and B are OK but

desk C does not have

adequate dayight

A B C

Externalstructure Section View

Fig. 4.9


9/18


________________________________________________________________________


As a general rule of thumb, the depth of a room should be limited to meet the

following condition:

L L 2

-- + --- < ----------

W H ( 1 - Rb)

Where L = Depth of a room

W = Width of a room

H = Height of the window head above the floor

Rb = Area weighted average reflectance in back half of

room (typically 0.5 for an office)

4.9 Calculation of Average Daylight Factor (D)

Visible sky

Fig. 4.7

W T

D = ---- ----------

A (1 - R2)

Where D = Average daylight factor

W = Window area in m2(use table to correct for framing)A = Area of all surfaces in the room in m2(floor, ceiling,

walls, and windows).

T = Glass transmittance (from table)

= Visible sky angle in degrees

R = average reflectance of (floor, ceiling, walls, and

windows).


10/18

Table 4.1 Correction factor for type of frame

Type of frame Cg

Metal patent glazing 0.9

Metal frame - large pane 0.8

Wood frame - large pane 0.7

Wood frame - small pane 0.6

Table 4.2 Correction factor for Glass transmission

Type of glass Ct

Clear 6mm single glazing 0.8

Clear 6mm double glazing 0.65

Tinted bronze 0.46

Tinted Grey 0.39

Tinted Green 0.66

Strongly reflecting 0.18

Table 4.3 Correction factor for Dirt on Glass

Location Cd

Clean 0.9

Industrial 0.7

Very dirty 0.6

4.11 Calculation of Daylight Factor at a point.

A sample point is normally selected in a room and a calculation is made of the

daylight factor at that point. Daylight factor is quoted as a percentage of the

outside illuminance. Computer programmes can calculate daylight factors at

various points in a given room with great ease and speed. Isolux diagrams of the

illuminance throughout the room can be produced once a value of external

daylight illuminance is input along with room and window data.

Daylight factor (D) is calculated as follows: D = Cg Ct Cd (Dc + De + Cr Di)

Where Dc = Direct sky component;De = Externally reflected component;

Di = Internally reflected component.

Cg = Correction factor for glazing bars which reduce glass area;

Ct = Correction factor for glazing materials other than clear

glazing;

Cd = Correction factor for dirt on windows;

Cr = Correction factor for dirt on internal surfaces.


11/18


________________________________________________________________________


4.11.1 Sky Component (Dc)

This is the light reaching a point in a room directly from the sky. The BRE

(building research establishment - in Britain) provide a sky component table

which is reproduced in the CIBSE code for Interior Lighting (Table 5.16 of

CIBSE Code for Interior Lighting).

Example 4.5

An office has a row of desks at cill height 2 metres from a window which is 1.2

metres high and 2.4 metres wide. Using table 4.5, calculate the direct sky

component:

(a) On the desk opposite the centre of the

window.

(b) On the desk opposite the side of the

window.

h = Height of window = 1.2m

d = Distance from window = 2m

W1 = Width of window to one side

W2 = Width of window to other side

Table 5.16

A

B

2m2.4m

Fig. 4.8


12/18

h 1.2

(a) --- = --- = 0.6

d 2

W1 1.2

---- = ----- = 0.6 (From Table Dc = 1.3%)

d 2

This is the percentage daylight factor from one side of window only. The benefit

from the other side is the same therefore.

Total Direct Component = 1.3% x 2 = 2.6%

h 1.2

(b) --- = ---- = 0.6

d 2

W 2.4

--- = ---- = 1.2 Therefore Dc = 1.9%

d 2

Example 4.6

Calculate the illuminance due to the direct sky component at points A and B in

Example 4.4 when the external horizontal illuminance is 10,000 lux.

(a) 10,000 x 2.6% = 260 lux

(b) 10,000 x 1.9% = 190 lux


13/18


________________________________________________________________________


Example 4.7

Calculate the direct sky component and the illuminance due to Dc if external

illuminance is 10 kilolux for points A, B and C. in Fig 4.7. The window is 1.2m

high.

At point A h = 1.2m; d = 1m; W1 = W2 = 3m

h 1.2 W 3

--- = --- = 1.2 ---- = ---- = 3

d 1 d 1

Dc = 6.2% x 2 = 12.4%

E due to direct sky component at point A = 10,000

x 12.4% = 1240 lux

At Point B h = 1.2m; d = 3m; W1 = 6m; W2 =0

h 1.2 W1 6

--- = --- = 0.4 --- = --- = 2

d 3 d 3

Dc = 0.96%

E due to direct sky component at point B = 10,000 x 0.96% = 96 lux

At Point C h = 1.2m; d = 3m; W1 = 4.5; W2 = 1.5m

h 1.2 W1 4.5

--- = ---- = 0.4 --- = ----- = 1.5

d 3 d 3

Dc = 0.95% due to benefit from one side

h 1.2 W2 1.5

--- = --- = 0.4 --- = --- = 0.5d 3 d 3

Dc total at point C = 0.95% + 0.54% = 1.49%

E due to direct sky component at point C

= 10,000 x 1.49% = 149 lux

6.0m1.0m

3.0m

4.5m

1.5m

Plan view

Fig. 4.9

A

B

C


14/18

Example 4.8

The examples considered so far have

calculated the sky component at cill height

opposite some part of the window. Consider

point P in Fig. 4.8. In order to calculate the

direct sky component at point P from windowQ, it is necessary to also consider wall areas P,

R and S. Treat these wall areas as though they

were windows as follows:

1. Calculate Dc for PQRS

2. Calculate and subtract Dc for PR

3. Calculate and subtract Dc for RS

4. Calculate and add Dc for R

Q = PQRS - PR - RS + R

1. PQRS h = 1.8 m; d = 1m ; W1 = 3.6m.

h 1.8 W 3.6

--- = --- = 1.8 --- = ---- = 3.6

d 1 d 1

Therefore Dc = 9.4%

2. PR h = 1.8m; d = 1m; W = 1.2m

h 1.8 W 1.2

--- = --- = 1.8 ---- = ----- = 1.2

d 1 d 1

Therefore Dc = 7.8%

3. RS h = 0.6m; d = 1m; W = 3.6m.

h 0.6 W 3.6--- = --- = 0.6 --- = ---- = 3.6

d 1 d 1

Therefore Dc = 2.1%

P

Q

R

S1.8m

1.2m

2.4m1.2m

0.6m

1.0m

Fig. 4.10

3.6m


15/18


________________________________________________________________________


4. R h = 0.6m; d = 1m; W = 1.2m.

h 0.6 W 1.2

--- = --- = 0.6 --- = ---- = 1.2

d 1 d 1

Therefore Dc = 1.9%

Dc at P = 9.4 - 7.8 - 2.1 + 1.9

= 1.4%

4.11.2 Externally Reflected Component (De)

This is the reflected daylight reaching a point in a roomfrom external structures and surfaces. It is only

necessary to calculate De if direct daylight is severely

limitedby an external structure.

It is seen from Fig. 4.9 that the external structure is

restricting the amount of daylight entering the window

W. In this case it will be necessary to calculate De.

Luminance of Obstruction

De = Dc x ---------------------------------

Luminance of Sky

For example if Dc = 2% and the luminance of the obstruction is one tenth of the

luminance of the sky.

1

Then De = 2% x --- = 0.2%

10

Fig. 4.11


16/18

4.11.3 Internally reflected component (Di)

This is the light reaching a point after reflection from surfaces within a room.

The amount of inter reflected light varies with the distance from the window but

the average internally reflected component for side lit rooms is:

0.85 AWDi = ------------(CRfw + 5 Rcw)

A(1-R)

Where:

W = area of windows;

A = area of ceiling, floor and walls (inc windows)

R = average reflectance of ceiling; floors and walls (inc windows)

Rfw = average reflectance of floor and walls below the plane of

mid height of the window (excluding window wall).

Rcw = average reflectance of ceiling and these parts of the wall

above mid height of window (exc window wall)C = coefficient dependent on the obstruction outside the window.

Values for C vary between 0 and 39, depending on the obstruction outside (39 is

used when there is no obstruction).

Angle of obstruction Coefficient C

No obstruction 39

10 degrees 35

20 degrees 31

30 degrees 25

40 degrees 20

50 degrees 14

60 degrees 10

70 degrees 7

80 degrees 5

Formulae and Tables supplied courtesy of BRE

4.11.4 Correction FactorThe same correction factors apply as for the calculation of average daylight

factor. (tables 4.1, 4.2 and 4.3 page 105)

4.11.5 Daylight Factor (D) This is the aggregate of the Direct Sky, the

Externally Reflected and the Internally Reflected components and can be

calculated as follows: D = Cg Ct Cd (Dc + De + Cr Di)

For most buildings, the direct sky component Dc is the most significant aspect of

daylight penetration.


17/18


________________________________________________________________________


Example 4.9.

A room measures 6.5m x 4m x and is 2.7m high. It contains one window

measuring 2m high by 2.5m wide located at the centre of the 4m wall. The cill

height is 0.5m. The window glass is clear with a transmittance factor of 0.8. The

minimum external horizontal illuminance due to daylight may be taken as 10,000

lux and there are no external obstructions.

Room reflectances are: Lower surfaces 0.4

Upper surfaces 0.6

Overall average 0.5

Calculate:

(a) The average daylight factor in the room

(b) The daylight factor and the daylight illuminance at a point in the

room located 5m normal to the centreline of the window at cillheight.

(c) The average daylight illuminance in the room

(d) Comment on the above results in relation to the design of the

artificial lighting as well as its control.

(a) W T

D = ---- ----------

A (1 - R2)

5 0.8 x 80

D = ------ ------------- = 3.92%

108.7 (1 - 0.52)

(b) Df = Dc + De + Di

De = 0

h 2 W 1.25Dc = --- = --- = 0.4 --- = ------ = 0.25

d 5 d 5

From Table 5.16 Dc = 0.295 x 2 = 0.59 %

0.8W

Di = ------------(CRfw + 5 Rcw)

A(1-R)


18/18

0.8 x 5

Di = ----------------(39 x 0.4 + 5 x 0.6) = 1.36%

108.7(1-0.5)

Total Df at point X = 0.59 + 1.36 = 1.95%

(c) Eav = 3.92% of 10,000 = 392 lux

E point x = 1.95% of 10,000 = 195 lux

(d) Permanent supplementary artificial lighting is required in rooms where the

average daylight is less than 5%. Modulation of artificial light will be desirable

to reduce energy costs. Automatic dimming could provide gradual control down

to 20% output. When daylight levels are high, automatic switching off of

artificial lighting would not be recommended for a room where the average

daylight factor is less than 5%.

2.1 daylighting h3

Documents